There is a known internal combustion engine which has a first fuel injection valve that injects fuel directly into a cylinder and a second fuel injection valve that injects fuel into an intake port, and which performs homogeneous combustion by supplying fuel into the cylinder through the use of these two fuel injection valves. In such internal combustion engines, generally, the combustion air-fuel ratio is adjusted to the fuel-lean side of the stoichiometry air-fuel ratio during a low engine load state, and is adjusted to the stoichiometric air-fuel ratio during a high engine load state. During the low engine load state, the proportion of the fuel injection from the second fuel injection valve is made larger than the proportion of the fuel injection from the first fuel injection valve, with the intention of further enhancing the homogeneity of homogeneous air-fuel mixture. During the high engine load state, the proportion of the fuel injection from the first fuel injection valve is made larger than the proportion of the fuel injection from the second fuel injection valve, with the intention of lowering the in-cylinder temperature and further enhancing the charging efficiency.
If the combustion air-fuel ratio is the stoichiometric air-fuel ratio, the combustion temperature becomes high, and thus heightens the in-cylinder temperature, so that deposit is likely to form on the nozzle hole of the first fuel injection valve that has an opening within the cylinder. Therefore, making the fuel injection proportion of the first fuel injection valve larger than the fuel injection proportion of the second fuel injection valve is advantageous for lowering the nozzle hole temperature of the first fuel injection valve and curbing the deposit precipitation on the nozzle hole.
In order to accurately control the amount of fuel supplied into a cylinder, it is necessary to correct the amount of fuel injected from each fuel injection valve. In the case of the aforementioned internal combustion engine, since fuel is always injected from the two fuel injection valves, it is difficult to set a different fuel injection correction coefficient for each fuel injection valve. Hence, the amount of fuel that is actually supplied into a cylinder is calculated from the air-fuel ratio of exhaust gas detected by an air-fuel ratio sensor. Then, on the basis of the excess or deficiency of the calculated fuel amount from a necessary fuel supply amount, the same fuel injection correction coefficient with respect to the two fuel injection valves is learned.
The thus-learned fuel injection correction coefficient is effective only with respect to the fuel injection proportion between the first fuel injection valve and the second fuel injection valve at the time of learning and, strictly speaking, the necessary fuel supply amount at the time of learning. Therefore, it has been proposed to learn a correction coefficient for each of operation regions of different fuel injection proportions (e.g., see Japanese Patent Application Publication No. JP-A-3-185242).
Generally, the air-fuel ratio sensor is able to detect an accurate air-fuel ratio near the stoichiometric air-fuel ratio. Therefore, during the homogenous combustion at the stoichiometric air-fuel ratio, a fuel injection correction coefficient with respect to the then used fuel injection proportion can be learned. However, the air-fuel ratio sensor is not able to accurately detect a ratio that is less than the air-fuel ratio of about 18, such as an air-fuel ratio occurring during the homogenous combustion at a lean air-fuel ratio for curbing the amount of NOX production. Therefore, during the lean air-fuel ratio homogenous combustion, an accurate fuel injection correction coefficient cannot be learned with respect to the then used fuel injection proportion. Furthermore, the air-fuel ratio sensor is also unable to accurately detect such a rich air-fuel ratio as in an operation (hereinafter, referred to as “rich spike”) in which the combustion air-fuel ratio is adjusted to the fuel-rich side to perform a regeneration process in which a NOX storage reduction catalyst disposed in the engine exhaust system is reduced and purified by releasing stored NOX therefrom. Therefore, during the rich spike, too, an accurate fuel injection correction coefficient cannot be learned with respect to the then fuel injection proportion.
Thus, during the lean air-fuel ratio homogenous combustion, the amount of fuel injection cannot be accurately corrected, so that a more-than-necessary amount of fuel may be supplied into a cylinder and the amount of NOX production may increase, or so that a less-than-necessary amount of fuel may be supplied into a cylinder and a necessary torque cannot be generated. Furthermore, during the rich spike, too, the amount of fuel injection cannot be accurately corrected, so that the regeneration process of the NOX storage reduction catalyst device may be performed insufficiently, or so that more fuel than needed for the regeneration process may be supplied and the fuel economy may deteriorate.